The present invention relates to a process for separating and cleaning coal of fine particle size and, more particularly, is concerned with a new and improved wet process for removing unwanted contaminant particles from fine coal utilizing mechanical and hydraulic means.
The invention has particularly application to the treatment of contaminated coal having a particle size of less than about 10 mm. Coal particles of small size are difficult and expensive to separate from the usual associated contaminant particles such as rock, pyrite, slime, clay, silt and the like. Moreover, with the increasing use of continuous mechanical coal mining apparatus, the generation of such finely sized coal by such extraction techniques has become a significant factor usually representing 30 to 40 percent of normal "run of mine" output.
One source of contaminated coal of such small particle size is in mining operations wherein the mined coal containing the usual rock and other contaminating impurities is separated into fractions by passing the crushed "run of mine" coal across a sizing screen deck while subjected to water sprays which assist in the separation. The subsequent cleaning of the large or coarse fraction containing particles having a size range above about 10 mm presents no problem as this can be readily accomplished using conventional mechanical cleaning methods or heavy media bath or drum separation techniques by means of which the coal particles are floated off while the contaminant material sinks and is removed as waste material. The fine fraction, however, is not susceptible to cleaning by this method, and requires a different treatment to permit the coal to be separated from its associated contaminants. Another potentially important source of contaminated coal of small particle size may be found in waste or refuse deposits produced during earlier mining operations when the fine material generally minus 6 mm or minus 0.6 mm in size, was regarded merely as waste material to be disposed of because there was at the time, little market demand and further treatment for recovery of the coal content was regarded as not economically feasible.
Various chemical and mechanical processes have been employed heretofore for cleaning contaminated coal particles of fine size. One of the better processes that has been commercially used is described and claimed in my prior U.S. Pat. No. 4,128,474 assigned to the assignee of the present invention and comprises passing a slurry of the fine particles through spiral gravity concentrators and separators to separate and remove the heavy contaminants following which the remaining fine coal slurry is delivered to a hydrocyclone separator to remove the very fine or ultra-fine contaminants. Other processes have included chemical flotation, shaking tables, heavy media cyclones, water only cyclones and air separation. Among these techniques, the most commonly used system has been the use of shaking table separators. The vibratory motion of the shaking tables has the disadvantages of poor efficiency below 48 mesh and high energy consumption. The shaking tables also require large plant floor space per ton processed and have a relatively high initial cost.
Another separating system widely used heretofore has been the use of heavy media cyclones. This process is described in detail in chapter 10 of the publication Coal Preparation published in 1968 by the American Institute of Mining, Metallurgical and Petroleum Engineers, Inc., New York, N.Y. In this process, finely sized coal is separated from its associated impurities by suspending the mixture of coal and the associated heavier than coal particles in a liquid medium composed of water and magnetite (iron particles) and then pumping this suspension to a hydrocyclone under pressure. The water/magnetite ratio is controlled so that the liquid medium exhibits an apparent specific gravity only slightly heavier than the coal being treated, thereby causing the coal particles to rise and overflow the cyclone while the heavier contaminants such as rock and pyrite sink within the cyclone and report as the cyclone underflow or refuse product.
While the use of heavy medium cyclones is a proven, highly efficient method for separating finely sized coal from heavier particles, it is not effective for treating materials having a size finer than about 28 mesh because the smaller impurity particles possess insufficient mass to sink within the dense liquid medium. Another disadvantage is the high cost of magnetite lost during processing because it cannot be effectively washed from the finely sized coal and refuse particles. This loss can amount to about 5 lbs. per ton of clean coal produced which in the operation of an average plant could result in an annual cost of as much as $100,000 for magnetite consumed based on current prices. Further disadvantages are high energy requirements and a relatively heavy investment in associated equipment such as draining and washing equipment for recovering and recycling the magnetite used in the process. Therefore, because of the desire to increase the availability and production of coal as an alternative economic source of energy to meet present energy requirements, the industry has been seeking to develop improved processes which are equally efficient and less expensive to install and operate for separating and cleaning coal particles of small size which typically account for about 1/4 to 1/3 of the total output of a modern mine that uses continuous mining machines.
Accordingly, it is a principal object of the present invention to provide a new process for separating and cleaning coal of fine particle size which is equally efficient in operation when compared to heretofore known methods and more economical to install and operate and which utilizes commercially available equipment.
A more specific object is to provide such an improved dense media separation process which does not require the use of expensive additive heavy media such as magnetite and which is characterized by a reduced energy consumption and does not require expensive ancillary equipment for magnetite media recovery.
Another object is to provide an improved process of the type referred to which is more versatile from the standpoint of being applicable for the processing of coal particles of fine size having a wide range of contaminant including particularly those with a very high refuse content and being controllable and easily variable for effecting a variety of separations.
Other objects will be in part obvious, and in part pointed out more in detail hereinafter.
These and related objects are accomplished in accordance with the present invention by providing an autogenous process for cleaning fine coal that utilizes a three, interdependent step technique for sequentially and selectively removing impurity fractions. In the first cleaning step, an aqueous slurry of the particles having a size of less than about 10 mm is fed to a hydrocyclone to remove the undesirable clay and silt particles smaller than about 0.1 mm together with most of the slime water present in the feed. In the second step, the hydrocyclone underflow containing the substantially deslimed particles of a size larger than about 0.1 mm is introduced to a dense medium separation vessel in which a dense fluidized bed of heavier than coal contaminant particles such as rock, shale and sandstone is maintained in such controlled fashion that coal particles entering the vessel are unable to penetrate the bed and instead, assisted by water which is injected into the vessel so as to cause a slight upward current flow, rises and overflows the vessel substantially free of the heavier contaminant particles originally present in the feed. In the third step, the water borne, coal rich overflow product recovered as just described and normally containing very fine contaminant material, typically less than about 0.250 mm in size, is fed to a static or vibrating screen in which the wire mesh or screen permits the water and very fine contaminants to pass through while the enriched dewatered coal product passes over the top of the deck to a collector as the desired finished coal end product.